U.S and coalition forces have increased air and artillery strikes against Islamic State fighters in Syria by more than 300 percent since March, a U.S. military official said Tuesday.

On Sunday, Syrian Democratic Forces launched the second phase of Operation Roundup to root out ISIS fighters from the middle Euphrates River valley and the Syria-Iraq border region, Col. Thomas Veale, a spokesman for Combined Joint Task Force-Operation Inherent Resolve, told reporters at the Pentagon.

Roundup, now in its 36th day, began May 1. Phase one of the operation ended May 14 when SDF forces secured the town of Baghuz. Phase two will focus on securing the town of Dashisha, he said.

Iraqi and coalition forces supported SDF forces with 225 cross-border air and artillery strikes in May.

"That's a 304 percent increase over March and a 123 percent increase over April," Veale said.

"We and our partners have pounded ISIS remnants from the ground and air in a relentless pursuit of their leaders and fighters," he said. "ISIS morale is low, and leaders are scurrying for their lives."

Veale emphasized the key role Iraqi Security Forces have played in the operation, in addition to providing security at the Iraq-Syria border.

"Iraqi forces have not simply been the anvil; they are also the hammer," he said.

The Iraqi Air Force conducted four strikes in Syria in April and May that demonstrated an "advanced Iraqi aviation planning capability that continues to get better every day," Veale said.

"These strikes took out key ISIS facilities in support of Operation Roundup; there will be more," he said. "Additionally, Iraqi artillery has also operated with coalition artillery in cross-border missions and will continue to do so until ISIS remnants are annihilated."